Here's a link to a basic set of interrogatories. It lets you see how the questions are worded and what to ask for, in general: http://www.burbageweddell.com/resources/sample-interrogatory-requests/
HOWEVER, you will need to adapt these questions to fit your facts and situation. By this, I mean that you will have to ask about the 401(k), where the money went, etc. You can ask for ANYTHING you want, and if the debtor objects, the judge can decide whether or not you're entitled to the information. But, there's no harm in asking!
Also, you can make requests for production of documents, and that is usually a simple request for him to produce ALL documents that he used or referred to in responding to the interrogatories. Here are some examples:
REQUEST NO. 1: Any and all documents supporting or in any manner relating
to your bankruptcy filing.
REQUEST NO. 2: Any and all valuation reports held by you in connection with the 401k account .
REQUEST NO. 3: Any and all documents which you intend to place into
evidence at the hearing of this adversary proceeding.
REQUEST NO. 4: Any and all documents that you relied on or refer to in responding to the interrogatories.
Here's a link as well: http://www.burbageweddell.com/resources/sample-request-for-production-of-documents-in-adversary-proceeding/
You can also issue requests for admissions, which basically asks the other party to agree with you that what you allege is true. Here's a good link to sample admissions: http://www.burbageweddell.com/resources/sample-requests-for-admission/